1. Field of the Invention
The present invention relates to a reflective control switchable thin film material and reflective control switchable (light control) glass in which a magnesium-titanium alloy thin film is used, and more particularly relates to a novel magnesium-titanium alloy-based reflective light control material used in light control glass that automatically controls the amount of sunlight coming through window glass, without using blinds or curtains, and to reflective light control glass produced using this material. The present invention provides window material technology pertaining to a novel reflective control switchable thin film material for controlling the transmissivity of sunlight through the windows of buildings or vehicles.
2. Description of the Related Art
Windows (openings) are generally the place where the most heat comes into and goes out of buildings. For example, about 48% of the heat produced by heating systems in winter escapes through windows, and the proportion of heat that comes in through the windows of air-conditioned rooms in summer can reach about 71%. Therefore, a tremendous energy savings can be realized by effectively controlling light and heat at windows. Light control glass was developed for this purpose, and its function is to control the inflow and outflow of light and heat.
There are several ways by which light is controlled with this light control glass. Of these, 1) a material that reversibly changes its transmissivity under application of current or voltage is called an electrochromic material, 2) a material whose transmissivity changes with temperature is called a thermochromic material, and 3) a material whose transmissivity is changed by controlling the atmosphere gas is called a gas chromic material. Of these, the most research has gone into electrochromic light control glass in which a tungsten oxide thin film is used for the light control layer, and this glass has just about reached the practical stage today, and some commercial products have already appeared.
Electrochromic light control glass that has been known up to now, including tungsten oxide, is all based on the principle that light is controlled by absorbing light with a light control layer. A drawback to this type of light control glass is that heat is produced when the light control layer absorbs light, and this heat is radiated back into the room, which diminishes the energy saving effect. To eliminate this drawback, the light control must be performed by reflecting light, rather than by absorbing it. Accordingly, there has been a need for the development of a light control material having characteristics that allow it to reversibly switch between a mirror state and a transparent state.
Such a material capable of switching between a mirror state and a transparent state could not be found for a long time, but in 1996 a group in the Netherlands discovered a hydride of a rare earth such as yttrium or lanthanum switches between a mirror state and a transparent state under the influence of hydrogen, and such a material has been termed a “switchable mirror” (J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijngaarden, J. P. Dekker, D. G. de Groot, N. J. Koeman, Nature, 380 (1996), 231). These rare earth hydrides undergo a large change in transmissivity, and have excellent light control mirror characteristics. Nevertheless, because a rare earth element is used in the material, there are problems in terms of resources and cost when these switchable mirrors are used for window coatings and so forth.
Materials known up to now to have reflective light control characteristics (switchable mirror characteristics) include hydrides of yttrium, lanthanum, and other rare earth metals, hydrides of allows of magnesium and a rare earth metal such as gadolinium, and hydrides of magnesium-nickel alloys. Of these, the ones most suited to coating window glass from the standpoint of resources and cost are those that make use of a magnesium-titanium alloy (Japanese Laid-Open Patent Application 2003-335553; Japanese Laid-Open Patent Application 2004-139134; Japanese Laid-Open Patent Application 2005-056706; Japanese Laid-Open Patent Application 2005-274630). However, light control layers made from materials reported up to now, while varying in degree, do not become colorless when transparent, and instead have a tint, which poses a major problem when applied to window glass.